1. Field of the Invention
The present invention relates to arrangements for sealing the floating roof of a liquid storage tank such as a gasoline storage tank, and more particularly to arrangements in which both primary and secondary seals are employed to seal the space between the floating roof and the inner wall of the tank.
2. History of the Prior Art
With increasingly stringent pollution standards being set in connection with the storage of gasoline and other volatile liquids, it has become commonplace to seal the space between a floating roof and the inner walls of a tank in which such liquids are stored. Examples of such seals are provided by U.S. Pat. Nos. 4,099,643 of Wardwell et al, 4,138,032 of McCabe, 4,130,217 of Hills et al, 3,735,891 of Nishkian et al, 3,167,206 of Nelson, 1,986,869 of Welp and 1,992,221 of Kramer.
The arrangement shown in the Wardwell et al patent includes a primary seal extending between the floating roof and the tank inner wall and a secondary seal which is mounted on the primary seal and which includes a plurality of wipers separated by spacer blocks. In the McCabe patent a primary seal combines with a secondary seal in the form of a single wiper having stiffeners mounted therein. In the arrangement shown in the Hills et al patent a primary seal is used in conjunction with a secondary seal supported by plates the top ends of which are disposed at or substantially in contact with the tank inner wall. The Nishkian et al patent shows a single seal which utilizes a tubular sealing member having a flanged wear cover and a flexible core member. The sealing arrangement shown in the Nelson patent includes a primary seal in combination with a secondary seal comprised of springs mounted on a fabric barrier. In the Welp patent a flexible sealing cover is disposed around the periphery of the main deck, beneath a number of segments which comprise a watershed. The arrangement shown in the Kramer patent uses a gas-tight fabric in conjunction with overlapping metal shoes engaging the side wall of the tank.
The various sealing arrangements of the prior art including those which employ both a primary seal and a secondary seal have been found incapable of meeting current evaporation and pollution standards in many instances. This is due to a number of factors which relate to the designs of such arrangements. In a typical installation, the primary seal substantially limits the escape of vapors from the liquid stored in the tank. However, the difficulty of providing a perfectly vapor-tight seal both at the juncture of the primary seal and the wall of the tank and at the juncture of the primary seal and the floating roof places a practical limit on the effectiveness of such a seal. Moreover, the body of the primary seal itself may leak due to minor damage or other deterioration with continued use. For this reason it has become commonplace to employ a secondary seal in conjunction with the primary seal, the secondary seal trapping the relatively small amount of vapors which get by the primary seal. However, most secondary seals are of limited effectiveness, due mostly to the difficulty in designing a different type of seal which must work in harmony with the primary seal. Of equal importance is the fact that the secondary seal must be capable of cooperating with the primary seal in accommodating the variety of different conditions occurring within the tank. In addition to variations introduced by climatic factors such as wind and temperature change, the tank itself may distort or undergo other aberrations making the continuous maintenance of a vapor-tight seal very difficult. For example, in the Wardwell et al patent the secondary seal which is dependent on the primary seal for its support and which itself is of rigid construction is not capable of conforming to the changing conditions so as to provide acceptable results in certain applications. Similar comments apply to McCabe where the single wiper element, although of relatively simple and inexpensive construction, is often incapable of functioning both as a vapor barrier and at the same time a sealing element to be maintained in contact at all times with the tank wall. In the Hills et al. arrangement the plates comprising the support structure for the secondary seal provide both a rigid and a flexible support but lack some means of maintaining an effective seal between their upper ends and the tank wall. In some arrangements such as that shown in the Nelson patent relatively fragile fabric is exposed to the weather and to direct sunlight, resulting in a greatly shortened useful life for such materials and the resultant leakage which occurs between the time such materials deteriorate or fail and are replaced.
Accordingly, it is an object of the invention to provide an improved sealing arrangement for use with tanks having a floating roof.
It is a further object of the invention to provide an improved secondary seal which is highly flexible and capable of conforming to changing shapes and sizes of the surrounding walls and parts.
It is a still further object to provide a secondary seal which in addition to being highly flexible and accommodating is arranged to protect the vapor-impermeable portions thereof against rapid wear and deterioration.